The real question is, is the 158 too big? If not, she should learn on a moderately large board with as small a kite that will keep her going, imo.
Donna learned on an 11m kite, and was going upwind (in as little as 11 knots) on a 158 Underground.

When it picked up, we put her on the 137 UG and just kept with the 11m, up to about 20. With a 12m Waroo (or similar) she could ride (no problem) in as little as 11 knots on your prodigy. Small boards are for being LITT imo, not necessarily for smaller people. Donna's smallest board is a custom UG (127 by 39), and she definitely doesn't need a smaller board.
To learn on, I think error on a tad big, this is FL after all, and we all need light wind boards anyway.